Assuming linux/apache, add this line to your .htaccess file (may vary depending on your php/apache config)
Code:
AddType application/x-httpd-php .html
I'm guessing you keep everything in a public_html folder, create a file called header.html in the public html folder, and put your shared header code in there.
HTML Code:
<meta name="keywords" content="your keywords" />
<meta name="description" content="your description" />
...
then, in every html page where you have the metadata, replace it with this line :
PHP Code:
<?php include ('/your/home/directory/public_html/header.html'); ?>
If you don't know your home directory, just wrap some gibberish in php tags and you can get the correct path from the error message it spits out. Also, if you're hosted with godaddy or one of their resellers you should be aware that they rotate their htaccess rules once an hour, so you'll have to wait an hour between testing the rules if something messes up.